1. Field of the Invention
This invention relate to a control system for an internal combustion engine which is provided with an evaporative emission control system for temporarily adsorbing evaporative fuel generated in the fuel tank of the engine and purging the same at suitable timing into the intake system of the engine, and more particularly to a control system of this kind, which carries out feedback control of the air-fuel ratio of an air-fuel mixture supplied to the engine by utilizing an adaptive control theory.
2. Prior Art
There is conventionally known an air-fuel ratio control system for internal combustion engines, for example, from Japanese Laid-Open Patent Publication (Kokai) No. 3-185244, in which an optimal regulator which is one of modern control theories is applied to air-fuel ratio feedback control such that the air-fuel ratio is feedback-controlled based on an output from a linear-output oxygen concentration sensor (LAF sensor) arranged in the exhaust system of the engine, and an optimum feedback gain calculated based on a dynamic model representative to the behavior of the engine.
Further, an evaporative fuel-processing system has been conventionally employed, in which evaporative fuel generated in the fuel tank is temporarily stored in a canister of the engine for purging to the intake system of the engine according to operating conditions of the engine.
According to the evaporative fuel-processing system, purging is carried out so as to exert the least possible influence upon air-fuel ratio control of an air-fuel mixture supplied to the engine. However, it is difficult to carry out purging by controlling the concentration of evaporative fuel being purged and the flow rate of a purged gas (mixture of evaporative fuel and air) with high accuracy. Further, if the air-fuel ratio is controlled by using a controller of a recurrence formula type as in the air-fuel ratio feedback control utilizing the optimal regulator, especially an adaptive controller having high deadbeat response, there is a fear that the controllability of the air-fuel ratio can be degraded when purging is carried out.
Besides, when the amount of purged fuel is large, the air-fuel ratio detected by an air-fuel ratio sensor shows a rich value, so that feedback control of the air-fuel ratio results in an extremely decreased amount of fuel injected from a fuel injection valve. Consequently, fuel injection is unfavorably carried out in a valve opening region where linearity between the valve opening period of the fuel injection valve and the fuel injection amount lowers to degrade the control accuracy of the air-fuel ratio control and hence spoil exhaust emission characteristics and drivability of the engine.